For example, in a process for producing semiconductor devices, various plasma processes, such as etching, sputtering and CVD (chemical vapor deposition), are used for processing semiconductor wafers serving as substrates to be processed.
As plasma processing systems for carrying out such plasma processes, various systems are used. Among these systems, capacitive coupled parallel plate plasma processing systems are mainly used.
The capacitive coupled parallel plate plasma processing system has a pair of parallel plate electrodes (top and bottom electrodes) in a chamber. This system is designed to introduce a process gas into the chamber and to apply high-frequency electric power to one of the electrodes to form a high frequency electric field between the electrodes to form plasma of the process gas by the high frequency electric field to plasma-process semiconductor wafers.
For example, when a film, e.g., an oxide film, on a semiconductor wafer is etched by means of such a capacitive coupled parallel plate plasma processing system, the optimum radical control can be carried out by causing the pressure in the chamber to be a medium pressure to form a medium density plasma. Thus, it is possible to obtain an appropriate plasma state, so that it is possible to realize stable and repeatable etching with a high etching selectivity.
However, in recent years, the scale down of the design rule for ULSIS is increasingly advancing, and a higher aspect ratio of a hole shape is required, so that conventional conditions are not always sufficient in etching of an oxide film and so forth.
Therefore, it has been attempted to raise the frequency of the high frequency electric power to be applied, to form a high density plasma while maintaining the good dissociation state of plasma. Thus, it is possible to form an appropriate plasma under lower pressure conditions, so that it is possible to appropriately cope with the further scale down of the design rule.
By the way, according to the results of the inventor's study, it was revealed that there was the following new problem when the frequency of the high frequency electric power to be applied was raised to increase the density of plasma.
Conventionally, the feed to the top electrode is carried out via a feeder rod. This feeder rod is provided at the center on the reverse surface of the top electrode. If the applied frequency is raised to form a high density plasma, the high frequency plasma only flows through the surface portion of the electrode. Thus, the high frequency electric power supplied from the feeder rod to the top electrode passes through the reverse surface of the electrode to reach the circumferential direction of the electrode to be gradually supplied along the plasma contact surface of the electrode from the circumferential side toward the center. In addition, the circumferential portion of the top electrode is surrounded by an insulator (capacitance component), and the chamber outside of the insulator is safety-grounded. Therefore, standing waves are formed on the plasma contact surface of the top electrode due to the interference action, so that the electric field distribution is non-uniform in radial directions of the electrode.
If the electric field distribution is thus non-uniform, the density of plasma is non-uniform, and the etching rate distribution is non-uniform in etching, so that it is required to remove the cause of such non-uniformity of the electric field distribution to make the etching rate distribution uniform.
However, conventionally, such problems caused by using the high density plasma have not been always recognized, and it has not sufficiently attempted to remove the above described non-uniformity of the electric field distribution in the present circumstances.